Friction stir welding (FSW) is a relatively new welding process for joining together parts of materials such as metals, plastics, and other materials that will soften and commingle under applied frictional heat to become integrally connected. A detailed description of an FSW apparatus and process may be found in Patent Publications WO 93/10935 and WO 95/26254; and U.S. Pat. No. 5,460,317, all of which are hereby fully incorporated by reference. One of the useful apparatus for FSW is shown in FIGS. 1A and 1B. As shown, two parts, exemplified by plates 10A' and 10B' on a backing plate 12', are aligned so that the edges of the plates to be welded together are held in direct contact. An FSW tool W' has a shoulder 14' at its distal end, and a nonconsumable welding pin 16' extending downward centrally from the shoulder. As the rotating tool W' is brought into contact with the interface between plates 10A' and 10B', the rotating pin 16' is forced into contact with the material of both plates, as shown. The rotation of the pin in the material and rubbing of the shoulder against the upper surface of the material produce a large amount of frictional heating of both the welding tool and the plate interface. This heat softens the material of the plates in the vicinity of the rotating pin and shoulder, causing commingling of material, which, upon hardening, forms a weld. The tool is moved longitudinally along the interface between plates 10A' and 10B', thereby forming an elongate weld along the interface between the plates. The welding tool's shoulder 14' prevents softened material from the plates from escaping upward, and forces the material into the weld joint. When the weld is completed, the welding tool is retracted.
As with any type of welding process, anomalies can occur in friction stir welding. The types of anomalies include circular voids, surface and subsurface longitudinal voids, and in-process exit holes. Circular and longitudinal voids occur due to parameter and size variations associated with friction stir welding, such as variations in length of the tool pin and thickness of the workpieces being welded. In-process exit holes are left from ending a friction stir weld prematurely. FIG. 2 illustrates a typical in-process exit hole 20 left unfilled from retracting welding tool W' before completing a weld along plates 10A' and 10B'. Resuming the welding process by simply welding over an in-process exit hole produces a void in the exit hole area due to lack of sufficient metal in the area. Rewelding over a circular void or a longitudinal void in the same manner also produces a secondary void. A need exists for a method to repair voids found in a friction stir weld without producing a secondary void. In particular, when multiple weld segments are to be used and overlap each other, voids found in the welds must be repaired as they occur, while in the welding mode. Accordingly, a need exists for an in-process method to repair voids in an aluminum alloy, particularly a friction stir weld, which utilizes equipment and fixtures being used for friction stir welding and, thus, does not require any additional tooling. The needed in-process repair method should repair the void without degrading metallurgical and mechanical properties of the material being repaired.